The invention relates to the failure of encapsulated circuits such as multi-chip modules resulting from contamination-induced physical deterioration of the conductor metal taking place within the package, generally after the device has been produced contaminant-free, but after being stored in a humid environment for a period of time. The most prevalent cause of such failure is the entry of moisture with dissolved ionic contaminants into the package.
The penetration of moisture into hermetically sealed electronic packages or the permeation of moisture through plastic in non-hermetically sealed packages, especially in the presence of small amounts of ionic contaminants (low parts per million of chloride ion, for example), is known to cause device or circuit failure. See for example, J. J. Licari, Handbook of Polymer Coatings For Electronics, (Noyes, 1990). The failure mechanisms are generally moisture and ion-induced chemical corrosion of a layer of thin film metallization, of aluminum, nichrome, or copper for example. These impurities cause electrochemical phenomena such as metal migration that catastrophically bridge and short out closely spaced conductors. Or, in bi-metallic wire bonds, such as gold to aluminum, resistances may increase and electrical opens occur while leakage currents increase. See G. Harman, Wire Bond Reliability and Yield, (ISHM Monograph, 1989), and J. J. Licari and L. Enlow, Hybrid Microcircuit Technology Handbook, (Noyes Publications, Park Ridge N.J., 1988).
It would be desirable to incorporate a moisture and contamination sensor in every multichip module so that the penetration of water through hermetically sealed or plastic-encapsulated electronic modules can be rapidly detected. Such a detector could be used to make "go-no go" decisions for modules that have been stored a long time in various harsh environments before deploying them in flight hardware and other critical applications, and would also be useful in deciding if modules already in service need to be replaced.
No such sensors exist today, despite considerable research and experimentation. Some tests destroy the modules, and most are awkward and time consuming to implement. To date, considerable work has been done on moisture sensors that are based on changes in the dielectric properties of thin film porous capacitor materials. This approach has been proven unsatisfactory because each chip had to be calibrated separately, and would then drift out of calibration after assembly and screen testing.
Currently used sensor chips such as Sandia Laboratory's ATC series are based on triple track resistors of thin film aluminum. However, they are not sensitive enough to show rapid resistance changes within or between the conductor lines, even after thousands of hours of biased 85/85 humidity/temperature exposure. Because of this, HAST (Highly Accelerated Stress Test) is being employed. However, it has been difficult to correlate HAST results with long term 85.degree. C. /85% relative humidity test results. Cynthia Murphy et al of Microelectronics Computer Technology Corporation (MCC) reported at the 1994 Multichip Module Conference in Denver that the 141.degree. C. temperature used by MCC in their HAST test was much too high for getting meaningful results on epoxy encapsulated chips. Degradation of the epoxy molecular structure occurred under HAST conditions which introduced new failure mechanisms and obfuscated any correlation with real-life expectancy. Although the Sandia chips are still excellent test sensors they are best used to initially qualify a material system or to obtain reliability data.
Special test chips have been designed to study moisture and ionic penetration onto active semiconductor devices based on changes in electrical resistance taking place due to corrosion occurring in thin film aluminum conductors (triple track resistors), or changes in capacitance that occur in porous oxide capacitors. (J. N. Sweet et al., Proceedings of the 41st Electronic Components and Technology Conference, 1991).
Other approaches depend on capacitance changes occurring in a capacitor device or a porous dielectric due to the condensation of moisture. These techniques require cooling a hermetically sealed package until the dew point of the internal ambient fluid is reached and the moisture condenses. U.S. Pat. No 4,224,656 to Sosniak and 4588943 to Hirth speak to this. Again these approaches require careful calibration of each chip, which is expensive to start with, and exposure to elevated temperatures throws off the settings and re-calibration is required. Still other approaches measure leakage currents or conductance of an integrated circuit, which has moisture condensing between the conductor lines resulting in a measurable increase in conductance due to the presence of the electrolyte.
Whatever the mechanism, it is particularly important to have some kind of moisture reporting device for electronics modules such as aircraft, missile or computer components, that have been stored for extended periods of time in warm, humid environments. Beyond military and high-tech applications and anything involving electronics, a suitable moisture sensor/indicator could also be used in any moisture-sensitive applications such as packaged commercial or consumer optical instruments, foods or pharmaceuticals.
There is a need for such a simple chip-mounted moisture sensing device that can serve a "go/no go" function, incorporated in multichip circuits to determine if moisture has penetrated and has affected, or has a high likelihood of affecting, the reliability of the modules, and which may be broader in its application to be useful in many situations in which the presence of moisture is of concern.